Method and apparatus for transmission and reception of in-band on-channel radio signals including complementary low density parity check coding

ABSTRACT

A method includes: constructing complementary low density parity check codewords by generating a first codeword having a first code rate; and partitioning the first codeword by assigning groups of bits of the first codeword to four quarter-partitions, wherein each of the quarter partitions includes bits in one half of one of four independently decodable semi-codewords each having a second code rate that is larger than the first code rate. Receivers that receive signals produced by the method are also disclosed.

CROSS REFERENCE TO A RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 13/835,011, filed Mar. 15, 2013, and titled “Method AndApparatus For Transmission And Reception Of In-Band On-Channel RadioSignals Including Complementary Low Density Parity Coding”, which ishereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for transmission andreception of radio signals in a digital radio broadcasting system.

BACKGROUND OF THE INVENTION

Digital radio broadcasting technology delivers digital audio and dataservices to mobile, portable, and fixed receivers. One type of digitalradio broadcasting, referred to as in-band on-channel (IBOC) digitalaudio broadcasting (DAB), uses terrestrial transmitters in the existingMedium Frequency (MF) and Very High Frequency (VHF) radio bands. HDRadio™ technology, developed by iBiquity Digital Corporation, is oneexample of an IBOC implementation for digital radio broadcasting andreception. IBOC DAB signals can be transmitted in a hybrid formatincluding an analog modulated carrier in combination with a plurality ofdigitally modulated carriers or in an all-digital format wherein theanalog modulated carrier is not used. Using the hybrid mode,broadcasters may continue to transmit analog AM and FM simultaneouslywith higher-quality and more robust digital signals, allowing themselvesand their listeners to convert from analog-to-digital radio whilemaintaining their current frequency allocations.

One feature of digital transmission systems is the inherent ability tosimultaneously transmit both digitized audio and data. Thus thetechnology also allows for wireless data services from AM and FM radiostations. The broadcast signals can include metadata, such as theartist, song title, or station call letters. Special messages aboutevents, traffic, and weather can also be included. For example, trafficinformation, weather forecasts, news, and sports scores can all bescrolled across a radio receiver's display while the user listens to aradio station.

The design provides a flexible means of transitioning to a digitalbroadcast system by providing three new waveform types: Hybrid, ExtendedHybrid, and All-Digital. The Hybrid and Extended Hybrid types retain theanalog FM signal, while the All-Digital type does not. All threewaveform types conform to the currently allocated spectral emissionsmask.

The digital signal is modulated using Orthogonal Frequency DivisionMultiplexing (OFDM). OFDM is a parallel modulation scheme in which thedata stream modulates a large number of orthogonal subcarriers, whichare transmitted simultaneously. OFDM is inherently flexible, readilyallowing the mapping of logical channels to different groups ofsubcarriers.

The HD Radio system allows multiple services to share the broadcastcapacity of a single station. One feature of digital transmissionsystems is the inherent ability to simultaneously transmit bothdigitized audio and data. Thus the technology also allows for wirelessdata services from AM and FM radio stations. First generation (core)services include a Main Program Service (MPS) and the StationInformation Service (SIS). Second generation services, referred to asAdvanced Application Services (AAS), include new information servicesproviding, for example, multicast programming, electronic programguides, navigation maps, traffic information, multimedia programming andother content. The AAS Framework provides a common infrastructure tosupport the developers of these services. The AAS Framework provides aplatform for a large number of service providers and services forterrestrial radio. It has opened up numerous opportunities for a widerange of services (both audio and data) to be deployed through thesystem.

The National Radio Systems Committee, a standard-setting organizationsponsored by the National Association of Broadcasters and the ConsumerElectronics Association, adopted an IBOC standard, designated NRSC-5A,in September 2005. NRSC-5A, the disclosure of which is incorporatedherein by reference, sets forth the requirements for broadcastingdigital audio and ancillary data over AM and FM broadcast channels. Thecurrent version of the standard is NRSC-5C, which is also herebyincorporated by reference. The standard and its reference documentscontain detailed explanations of the RF/transmission subsystem and thetransport and service multiplex subsystems.

SUMMARY

In a first aspect, the invention provides a method of transmittingdigital information, including: constructing complementary low densityparity check codewords by generating a first codeword having a firstcode rate; and partitioning the first codeword by assigning groups ofbits of the first codeword to four quarter-partitions, wherein each ofthe quarter partitions includes bits in one half of one of fourindependently decodable semi-codewords each having a second code ratethat is larger than the first code rate.

In another aspect, the invention provides a method including:constructing complementary low density parity check codewords bygenerating a first semi-codeword including information bits and firstparity bits; permuting and re-encoding the information bits of the firstsemi-codeword to produce second parity check bits and forming a secondsemi-codeword including the information bits and second parity bits;generating a third semi-codeword from the information bits of the firstsemi-codeword plus a first half of the parity bits from each of thefirst semi-codeword and the second semi-codeword; and generating afourth semi-codeword from the information bits of the firstsemi-codeword plus a second half of the parity bits from each of thefirst semi-codeword and the second semi-codeword.

In another aspect, the invention provides a receiver for receiving adigital radio signal. The receiver includes an input for receiving aradio signal including at least one carrier, the carrier being modulatedby plurality of information bits representing audio information and/ordata encoded in a composite codeword and a plurality of complementarylow density parity check codewords including a first codeword having afirst code rate; wherein the first codeword is partitioned by in groupsof bits of the first codeword in four quarter-partitions, and whereineach of the quarter partitions includes bits in one half of one of fourindependently decodable semi-codewords each having a second code ratethat is larger than the first code rate; and a processor for producingan output signal in response to the received radio signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmission system for use in an in-bandon-channel digital radio broadcasting system.

FIG. 2 is a schematic representation of a hybrid FM IBOC waveform.

FIG. 3 is a schematic representation of another hybrid FM IBOC waveform.

FIG. 4 is a schematic representation of another hybrid FM IBOC waveform.

FIG. 5 is a diagram that illustrates a portion of the signal processingin an FM IBOC transmitter.

FIG. 6 is a schematic representation of an example of codewordpartitioning.

FIG. 7 is a block diagram of an example of complementary low densityparity check (CLDPC) forward error correction (FEC) signal flow.

FIG. 8 is a schematic representation of another example of codewordpartitioning.

FIG. 9 is a block diagram of another example of CLDPC FEC signal flow.

FIG. 10 is a schematic representation of another example of codewordpartitioning.

FIG. 11 is a block diagram of another example of CLDPC FEC signal flow.

FIG. 12 is a schematic representation of another example of codewordpartitioning.

FIG. 13 is a block diagram of another example of CLDPC FEC signal flow.

FIG. 14 is a schematic representation of another example of codewordpartitioning.

FIG. 15 is a block diagram of another example of CLDPC FEC signal flow.

FIG. 16 is a schematic representation of another example of codewordpartitioning.

FIG. 17 is a functional block diagram of an FM IBOC DAB receiver.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 is a functional block diagram of therelevant components of a studio site 10, an FM transmitter site 12, anda studio transmitter link (STL) 14 that can be used to broadcast an FMIBOC signal. The studio site includes, among other things, studioautomation equipment 34, an Ensemble Operations Center (EOC) 16 thatincludes an importer 18, an exporter 20, an exciter auxiliary serviceunit (EASU) 22, and an STL transmitter 48. The transmitter site includesan STL receiver 54, a digital exciter 56 that includes an exciter engine(exgine) subsystem 58, and an analog exciter 60. While in FIG. 1 theexporter is resident at a radio station's studio site and the exciter islocated at the transmission site, these elements may be co-located atthe transmission site.

At the studio site, the studio automation equipment supplies mainprogram service (MPS) audio 42 to the EASU, MPS data 40 to the exporter,supplemental program service (SPS) audio 38 to the importer, and SPSdata 36 to the importer. MPS audio serves as the main audio programmingsource. In hybrid modes, it preserves the existing analog radioprogramming formats in both the analog and digital transmissions. MPSdata, also known as program service data (PSD), includes informationsuch as music title, artist, album name, etc. Supplemental programservice can include supplementary audio content as well as programassociated data.

The importer contains hardware and software for supplying advancedapplication services (AAS). A “service” is content that is delivered tousers via an IBOC broadcast, and AAS can include any type of data thatis not classified as MPS, SPS, or Station Information Service (SIS). SISprovides station information, such as call sign, absolute time, positioncorrelated to GPS, etc. Examples of AAS data include real-time trafficand weather information, navigation map updates or other images,electronic program guides, multimedia programming, other audio services,and other content. The content for AAS can be supplied by serviceproviders 44, which provide service data 46 to the importer via anapplication program interface (API). The service providers may be abroadcaster located at the studio site or externally sourced third-partyproviders of services and content. The importer can establish sessionconnections between multiple service providers. The importer encodes andmultiplexes service data 46, SPS audio 38, and SPS data 36 to produceexporter link data 24, which is output to the exporter via a data link.

The exporter 20 contains the hardware and software necessary to supplythe main program service and SIS for broadcasting. The exporter acceptsdigital MPS audio 26 over an audio interface and compresses the audio.The exporter also multiplexes MPS data 40, exporter link data 24, andthe compressed digital MPS audio to produce exciter link data 52. Inaddition, the exporter accepts analog MPS audio 28 over its audiointerface and applies a pre-programmed delay to it to produce a delayedanalog MPS audio signal 30. This analog audio can be broadcast as abackup channel for hybrid IBOC broadcasts. The delay compensates for thesystem delay of the digital MPS audio, allowing receivers to blendbetween the digital and analog program without a shift in time. In an AMtransmission system, the delayed MPS audio signal 30 is converted by theexporter to a mono signal and sent directly to the STL as part of theexciter link data 52.

The EASU 22 accepts MPS audio 42 from the studio automation equipment,rate converts it to the proper system clock, and outputs two copies ofthe signal, one digital (26) and one analog (28). The EASU includes aGPS receiver that is connected to an antenna 25. The GPS receiver allowsthe EASU to derive a master clock signal, which is synchronized to theexciter's clock by use of GPS units. The EASU provides the master systemclock used by the exporter. The EASU is also used to bypass (orredirect) the analog MPS audio from being passed through the exporter inthe event the exporter has a catastrophic fault and is no longeroperational. The bypassed audio 32 can be fed directly into the STLtransmitter, eliminating a dead-air event.

STL transmitter 48 receives delayed analog MPS audio 50 and exciter linkdata 52. It outputs exciter link data and delayed analog MPS audio overSTL link 14, which may be either unidirectional or bidirectional. TheSTL link may be a digital microwave or Ethernet link, for example, andmay use the standard User Datagram Protocol or the standard TCP/IP.

The transmitter site includes an STL receiver 54, an exciter 56 and ananalog exciter 60. The STL receiver 54 receives exciter link data,including audio and data signals as well as command and controlmessages, over the STL link 14. The exciter link data is passed to theexciter 56, which produces the IBOC waveform. The exciter includes ahost processor, digital up-converter, RF up-converter, and exginesubsystem 58. The exgine accepts exciter link data and modulates thedigital portion of the IBOC waveform. The digital up-converter ofexciter 56 converts from digital-to-analog the baseband portion of theexgine output. The digital-to-analog conversion is based on a GPS clock,common to that of the exporter's GPS-based clock derived from the EASU.Thus, the exciter 56 includes a GPS unit and antenna 57. The RFup-converter of the exciter up-converts the analog signal to the properin-band channel frequency. The upconverted signal is then passed to thehigh power amplifier 62 and antenna 64 for broadcast. In an AMtransmission system, the exgine subsystem coherently adds the backupanalog MPS audio to the digital waveform in the hybrid mode; thus, theAM transmission system does not include the analog exciter 60. Inaddition, the exciter 56 produces phase and magnitude information andthe analog signal is output directly to the high power amplifier.

Signal processing in both transmitters and receivers of an IBOC radiosystem can be implemented using a multi-layer logical protocol stack. Anexample of a logical protocol stack is shown in U.S. Pat. No. 8,111,716,which is hereby incorporated by reference. The signal processingdescribed in FIG. 5 below can be performed in Layer 1 (i.e., thephysical layer) of the logical protocol stack.

IBOC signals can be transmitted in both AM and FM radio bands, using avariety of waveforms. The waveforms include an FM hybrid IBOC waveform,an FM all-digital IBOC waveform, an AM hybrid IBOC waveform, and an AMall-digital IBOC waveform.

FIG. 2 is a schematic representation of a hybrid FM IBOC waveform 70.The waveform includes an analog modulated signal 72 located in thecenter of a broadcast channel 74, a first plurality of evenly spacedorthogonally frequency division multiplexed subcarriers 76 in an uppersideband 78, and a second plurality of evenly spaced orthogonallyfrequency division multiplexed subcarriers 80 in a lower sideband 82.The digitally modulated subcarriers are divided into partitions andvarious subcarriers are designated as reference subcarriers. A frequencypartition is a group of 19 OFDM subcarriers containing 18 datasubcarriers and one reference subcarrier.

The hybrid waveform includes an analog FM-modulated signal, plusdigitally modulated primary main subcarriers. The subcarriers arelocated at evenly spaced frequency locations. The subcarrier locationsare numbered from −546 to +546. In the waveform of FIG. 2, thesubcarriers are at locations +356 to +546 and −356 to −546. Each primarymain sideband is comprised of ten frequency partitions. Subcarriers 546and −546, also included in the primary main sidebands, are additionalreference subcarriers. The amplitude of each subcarrier can be scaled byan amplitude scale factor.

FIG. 3 is a schematic representation of an extended hybrid FM IBOCwaveform 90. The extended hybrid waveform is created by adding primaryextended sidebands 92, 94 to the primary main sidebands present in thehybrid waveform. One, two, or four frequency partitions can be added tothe inner edge of each primary main sideband. The extended hybridwaveform includes the analog FM signal plus digitally modulated primarymain subcarriers (subcarriers +356 to +546 and −356 to −546) and some orall primary extended subcarriers (subcarriers +280 to +355 and −280 to−355).

The upper primary extended sidebands include subcarriers 337 through 355(one frequency partition), 318 through 355 (two frequency partitions),or 280 through 355 (four frequency partitions). The lower primaryextended sidebands include subcarriers −337 through −355 (one frequencypartition), −318 through −355 (two frequency partitions), or −280through −355 (four frequency partitions). The amplitude of eachsubcarrier can be scaled by an amplitude scale factor.

FIG. 4 is a schematic representation of an extended hybrid FM IBOCwaveform 100. The extended hybrid waveform is created by adding primaryextended sidebands 102, 104 to the primary main sidebands present in thehybrid waveform. One, two, or four frequency partitions can be added tothe inner edge of each primary main sideband. The extended hybridwaveform includes the analog FM signal plus digitally modulated primarymain subcarriers (subcarriers +356 to +546 and −356 to −546) and some orall primary extended subcarriers (subcarriers +318 to +355 and −318 to−355).

The upper primary extended sidebands include subcarriers 337 through 355(one frequency partition), or 318 through 355 (two frequencypartitions). The lower primary extended sidebands include subcarriers−337 through −355 (one frequency partition), or −318 through −355 (twofrequency partitions). The amplitude of each subcarrier can be scaled byan amplitude scale factor.

In each of the waveforms of FIGS. 2-4, the digital signal is modulatedusing orthogonal frequency division multiplexing (OFDM). OFDM is aparallel modulation scheme in which the data stream modulates a largenumber of orthogonal subcarriers, which are transmitted simultaneously.OFDM is inherently flexible, readily allowing the mapping of logicalchannels to different groups of subcarriers.

In the illustrated hybrid waveforms, the digital signal is transmittedon a plurality of subcarriers in sidebands on either side of the analogFM signal. The power level of each sideband is appreciably below thetotal power in the analog FM signal. The analog signal may be monophonicor stereo, and may include subsidiary communications authorization (SCA)channels.

A Forward Error Correction (FEC) technique for an IBOC (In-BandOn-Channel) digital radio system is presented here. This FEC code isreferred to as Complementary Low-Density Parity Check (CLDPC) coding.The CLDPC coding is designed to accommodate the likely interferencescenarios encountered in the IBOC broadcast channel.

Low Density Parity Check codes are a type of large block forward errorcorrection (FEC) code. Because of their large size (typically manythousands of bits in each codeword), their error-correcting performancecan approach the theoretical (Shannon) limit. It has been well-knownthat very large codes can approach this theoretical performance limit,but common techniques of encoding and decoding these codes were believedto be impractical before a few decades ago, when a practical iterativedecoding technique for a specially constructed LDPC code was discovered.This suboptimum iterative decoding technique was based on “beliefpropagation”, and the complexity was dramatically reduced by requiring asparse parity check matrix along with some rules to ensure that allinput information bits were sufficiently represented in the sparseparity checks. It is now well-known that this LDPC code with itsconstruction restrictions, low-density, and simple suboptimum iterativedecoding rules can still achieve performance near the theoreticallimits. The performance and iterative nature of LDPC codes is similar toanother class of iterative convolutional codes called “Turbo Codes”.

LDPC codes are developed using various methods. There are regular andirregular LDPC codes created using various construction techniques.There are systematic and nonsystematic LDPC codes, where systematiccodes contain all the original information bits in the codeword, as wellas additional parity check code bits, and nonsystematic LDPC codes arecomprised of only parity check code bits. Less common, an LDPC code canbe constructed as a convolutional code, and can be terminated as ablock, and decoded using a tail-biting termination. The convolutionalLDPC code has similar performance to the block code, but is somewhatmore flexible in the size of the coded block, as it can be terminated atan almost arbitrary size. The LDPC, block code size is fixed for eachcode (parity check matrix), whereas the convolutional LDPC code size canbe terminated at different lengths without changing the generator(parity checks), All of these LDPC, code construction methods can beapplicable to the CLDPC code described herein.

Both frequency and time diversity can be exploited in the IBOCtransmitted signal to yield high performance in the presence ofinterference or fading. The digital signal modulation technique is codedorthogonal frequency division multiplexing (COFDM). This techniqueplaces a number of narrowband subcarriers on either end of the channelbandwidth, resulting in a Lower Sideband (LSB) and an Upper Sideband(USB). For time diversity, the information that is transmitted on theIBOC signal is divided into a Main channel and a Backup channel. TheMain and Backup channels are separated in time. For frequency diversity,the Main and Backup channel information is transmitted on subcarriers inboth the Lower and Upper Sidebands.

Since either the LSB or USB can be corrupted by a first adjacent signal(due to spectral crowding and frequency allocations in adjacent cities),the code used to transmit the Main and Backup channel information shouldpermit sufficient decoding on one sideband when the other is corrupted.The same information is carried on each sideband, although they each usehalf of the CLDPC block code, and the corrupted half is effectivelypunctured. The half-size block code on each sideband is defined as asemi-codeword. The initial C in CLDPC refers to the complementaryproperties of the pair of semi-codewords comprising a composite CLDPCcodeword. So each semi-codeword must also be a good LDPC code on itsown. When both sidebands are available, the composite CLDPC codeword ismore powerful than the semi-codewords. The full CLDPC code spanning bothsidebands has a 3-dB energy advantage over one sideband, plus additionalcoding gain if the semi-codewords are not exact duplicates. Of course,sideband diversity gain is also an advantage in nonuniform interference,as well as frequency selective fading.

Time diversity can also be exploited to improve performance in fadingand accommodate short signal blockages (e.g., traveling under a bridge).Time diversity is partially accomplished through block interleaving.However, some additional advantages are realized if the CLDPC code isagain segmented into two or more semi-codewords. One semi-codeword(representing the Main channel information, defined as the Mainsemi-codeword, and abbreviated as M) is transmitted first with arelatively long interleaver, while the second semi-codeword(representing the Backup channel information, defined as the Backupsemi-codeword, and abbreviated as B) is transmitted after severalseconds with a shorter interleaver. This short interleaver should be onthe order of 100 msec (instead of several seconds) to minimize tuningtime and accommodate fast acquisition on the backup channel. This timediversity offers improved performance in fading, where both Main andBackup semi-codewords are less likely to be simultaneously corrupted. Italso provides protection when losing the signal while a car is travelingunder a bridge. A complete signal outage of less than the duration ofthe diversity delay can be tolerated. Additional semi-codewords L(representing information to be transmitted on the lower sideband) and U(representing information to be transmitted on the upper sideband) aredescribed below.

The basic requirements for the CLDPC code include the ability toseparate the original code in various, possibly overlapping, codepartitions, where each partition comprises a semi-codeword.Semi-codewords are defined as Main, Backup, Lower and Upper (referred toherein as M, B, L and U partitions). Overlapping code partitions arecode partitions that include at least some common bits. Each of thepartitions must survive as a good code.

It is important to optimize the performance of the Main and Backuppartitions as a pair of approximately symmetric, complementarynon-overlapping partitions. Non-overlapping partitions are partitionsthat do not contain common bits. It is also important to optimize theperformance of the Lower and Upper Sidebands as a pair of approximatelysymmetric, complementary non-overlapping partitions when both sidebandsare used.

Of course, all code partitions should be noncatastrophic codes.

In one embodiment, the CLDPC codes can be created through partitioningof a longer (lower rate, e.g., R=⅓) LDPC code. However, it is alsopossible to Create this lower rate code by constructing it from shorter(higher rate) codes, to ensure that it can be partitioned. The FEC coderequires appropriate puncture patterns to provide good results. Thepuncture pattern would provide code bits for the upper sideband andlower sideband complementary components. Each sideband is required toprovide a good quality code in the case of the other sideband beingcorrupted. The code must also be partitioned for diversity with Main andBackup components. Each complementary component will be coded using ahigher rate (e.g., rate ⅔) code, producing a lower combined code rate(of ⅓, for example).

The FEC puncture pattern is distributed between a Main channel and aBackup channel. The Backup channel is used for fast tuning and providestime diversity to mitigate the effects of intermittent blockages. FIG. 5is a diagram that illustrates a portion of the signal processing in anFM IBOC transmitter. Audio and data information are input as illustratedby arrow 110. This information is assembled into frames of audio packetsand data as shown in block 112. Reference carriers (as described in U.S.Pat. No. 7,305,056 for “Coherent Tracking For FM In-Band On-ChannelReceivers”) are generated as shown in block 114 and synchronized withthe audio and data frames. The information in the audio and data framescan be scrambled as shown in block 116, and optionally encoded andinterleaved as shown in block 118 (if an outer code such as a ReedSolomon code is desired). This produces k information (plus optional RS)bits/codeword that are input to CLDPC encoder 120.

The CLDPC encoder processes the incoming bits to produce a plurality ofsemi-codewords having n bits/codeword. These CLDPC semi-codewordsrepresent information to be transmitted on main M and backup Bpartitions, as well as the U and L partitions.

Groups of bits of the semi-codeword are assigned to a backup upperquarter-partition BU, a backup lower quarter-partition BL, a main upperquarter-partition MU and a main lower quarter-partition ML. Bits in thelower quarter-partitions are to be transmitted on subcarriers in thelower sideband of the IBOC signal and bits in the upperquarter-partitions are to be transmitted on subcarriers in the uppersideband of the IBOC signal. The upper backup and lower backupquarter-partitions are delayed 122 with respect to the upper main andlower main quarter-partitions, and interleaved as shown in block 124.The interleaved BL quarter-partition is mapped 126 to OFDM subcarriersin a lower sideband of the in-band on-channel radio signal, and theinterleaved BU information is mapped 128 to OFDM subcarriers in an uppersideband of the in-band on-channel radio signal.

The main information is interleaved as shown in block 130. Theinterleaved ML quarter-partition is mapped 126 to OFDM subcarriers in alower sideband of the in-band on-channel radio signal, and theinterleaved MU quarter-partition is mapped 128 to OFDM subcarriers in anupper sideband of the in-band on-channel radio signal. The lowersideband symbols are then delivered on lines 132 and the upper sidebandsymbols are delivered on lines 134. The subcarriers are modulated andsubject to peak-to-average power ratio reduction as shown in block 136to produce a signal on line 138 that can be upconverted for transmissionas an in-band on-channel radio signal, using for example, thetransmitter shown in FIG. 1.

A CLDPC composite codeword is partitioned into independently-decodable“semi-codewords”. The semi-codewords are separated intoquarter-partitions where corruption due to interference and fading istypically uncorrelated. This is preferred over random interleaving (nopartitioning) because semi-codewords are also good LDPC codes.Soft-decision decoding can be used for the entire codeword, whilecorrupted components are appropriately weighted or erased.

This allows decoding under most likely impaired conditions leaving atleast one viable semi-codeword. The composite codeword has additionalenergy and coding gain when both semi-codewords are available.

There are generally 2 techniques for creating the CLDPC code. The firsttechnique starts with a low-rate codeword (e.g., R=⅓). This low-ratecodeword is then partitioned into semi-codewords (which can bedesignated as L, U, M and B) such that the semi-codewords (e.g., R=⅔)are independently decodable with “good” performance. Groups of bits ofthe semi-codewords are then assigned to 4 quarter-partitions: BL, BU, MLand MU, where L=BL+ML, U=BU+MU, M=ML+MU, and B=BL+BU. Since thesemi-codewords are not generally systematic, performance of thesemi-codewords should be checked. This technique should result in CLDPCcodes having the highest combined performance, but with semi-codewordsthat may not be optimum because they are restricted by partitioning.Systematic CLDPC codes contain the information bits in each codeword,whereas nonsystematic CLDPC codes are comprised only of parity checkbits. The performance of semi-codewords can be checked using, forexample, a computer search.

The second technique starts by generating a B semi-codeword. The sameinformation bits are then permuted and re-encoded to produce new paritycheck bits to form a complementary M semi-codeword. An L semi-codewordis then generated from the original information bits plus half of theparity bits from each B and M. Next, a U semi-codeword can be generatedfrom the original information bits plus the remaining half of the paritybits, or just parity bits for a nonsystematic code. Then performance ofL and U is verified, and parity bits are repartitioned if performance isnot acceptable. This should result in CLDPC codewords having the highestsemi-codeword performance for B and M, but the combined performance isnot optimum because the information bits occur twice. Constructiontechniques used to create L and U can be swapped with techniques forcreating B and M if single-sideband corruption is more likely thantemporal outages. Systematic CLDPC codes contain the information bits ineach codeword, whereas nonsystematic CLDPC codes are comprised only ofparity check bits

FIG. 6 is a schematic representation of an example of codewordpartitioning of a rate ⅓ CLDPC code using Technique 1. In this example,a codeword 150 includes 4608 information bits and 9216 parity checkbits. The information bits are divided into four groups of 1152 bitseach. The parity bits are divided Into four groups of 2304 bits each.One of the groups of information bits and one of the groups of paritycheck bits are mapped to each BL (backup lower), BU (backup upper), ML(main lower) and MU (main upper) quarter-partition. The bits areselected such that they form nonsystematic rate ⅔ non-catastrophicsemi-codewords identified as bits encompassed by ovals 152, 154, 156 and158. These semi-codewords can be transmitted in the in-band on-channelradio signal illustrated in FIG. 4, which includes 84-kHz upper andlower sidebands, each having 229 subcarriers. The information throughputis approximately 100 kbps. The semi-codewords can be combined at areceiver to produce a composite rate ⅓ codeword. In this example,semi-codeword L includes one half of the bits of each of semi-codewordsB and M, and semi-codeword U includes the other half of the bits of eachof semi-codewords B and M.

FIG. 6 also shows 2-dimensional overlapping codeword partitioning.Frequency diversity is provided by the upper and lower sidebands, thattransmit semi-codewords U and L. Time diversity is provided byMain/Backup semi-codewords M and B. BL, BU, ML and MU representoverlapped quarter-partitions. Each quarter-partition contains half of 2semi-codewords. Each codeword bit is assigned to BL, BU, ML or MU. Thecomposite codewords are composed of semi-codewords as: B=BL+BU, M=ML+MU,L=BL+ML, and U=BU+MU. Semi-codeword B can be transmitted after M withabout a 4 sec diversity delay. The semi-codeword B is comprised of BLand BU.

FIG. 7 is a block diagram of an example of CLDPC forward errorcorrection (FEC) signal flow. In this example, 4608 information bits areinput on line 160 and are encoded using a rate ⅓ code generator 162 toproduce a CLDPC codeword having 13824 bits on line 164. The CLDPCcodeword is demultiplexed 166 to produce the four groups of informationbits and parity check bits illustrated in FIG. 6. These four groups ofbits are multiplexed 168 to produce four groups of bits on lines 170,172, 174 and 176 that are mapped to the BL, BU, ML and MUquarter-partitions. Diversity delays 178 and 180 are added to the bitsfor the BU and BL partitions.

FIG. 8 is a schematic representation of another example of codewordpartitioning using a rate ½ CLDPC code (plus redundant information bits)using technique 2. In this example, a codeword 190 includes 4608information bits and 4608 parity check bits. The information bits aredivided into two groups of 2304 bits each. The parity bits are dividedinto four groups of 1152 bits each. One of the groups of informationbits and one of the groups of parity check bits are mapped to each BL,BU, ML and MU quarter-partitions. Notice that the two groups of 2304information bits are each redundantly mapped to two quarter-partitions.The bits are selected and mapped such that they form systematic rate ⅔non-catastrophic semi-codewords identified as bits encompassed by ovals192, 194, 196 and 198. These semi-codewords can be transmitted in thein-band on-channel radio signal illustrated in FIG. 4, which includes 84kHz upper and lower sidebands, each having 229 subcarriers. Theinformation throughput is approximately 100 kbps. The semi-codewords canbe combined at a receiver to produce a composite rate ⅓ codeword.

FIG. 9 is a block diagram of an example of CLDPC forward errorcorrection (FEC) signal flow. In this example, 4608 information bits areinput on line 200 and are encoded using a rate ½ code generator 202 toproduce a CLDPC codeword having 9216 bits on line 204. The CLDPCcodeword is demultiplexed 206 to produce the four groups of informationbits and parity check bits illustrated in FIG. 8. These four groups ofbits are multiplexed into quarter partitions 208 to produce four groupsof bits on lines 210, 212, 214 and 216 that are mapped to the BL, BU, MLand MU quarter-partitions. Diversity delays 218 and 220 are added to thebits for the BU and BL quarter-partitions.

FIG. 10 is a schematic representation of another example of codewordpartitioning using a rate ½ CLDPC code. This is very similar to thecodeword partitioning of FIG. 8, except in the way that the parity bitsare generated. We start by generating an R=⅔ CLDPC codeword, and thencreate a second CLDPC codeword by simply permuting the information bitsto form a new set of parity check bits. This results in a second R=⅔CLDPC codeword with redundant information bits. The result is nearly thesame as the partitioning of FIG. 8. The advantage here is that we needto define only a single good R=⅔ LDPC code, since the second codeword iscreated through permutation of the replicated information bits. In thisexample, a codeword 230 includes 4608 information bits and two groups of2304 parity check bits. The information bits are divided into two groupsof 2304 bits each. The parity bits are divided into four groups of 1152bits each. One of the groups of information bits and one of the groupsof parity check bits are mapped to each BL, BU, ML and MUquarter-partitions. Notice that the two groups of 2304 information bitsare each redundantly mapped to two quarter-partitions. The bits areselected such that they form systematic rate ⅔ non-catastrophicsemi-codewords identified as bits encompassed by ovals 232, 234, 236 and238. These semi-codewords can be transmitted in the in-band on-channelradio signal illustrated in FIG. 4, which includes 84 kHz upper andlower sidebands, each having 229 subcarriers. The information throughputis approximately 100 kbps. The semi-codewords can be combined at areceiver to produce a composite rate ⅓ codeword.

FIG. 11 is a block diagram of an example of CLDPC forward errorcorrection (FEC) signal flow. In this example, 4608 information bits areinput on line 250 and are encoded using a rate ⅔ code generator 252 toproduce a CLDPC codeword having 6912 bits on line 254. The informationbits are also permutated as shown in block 256 and encoded using the ⅔code generator to produce 2304 parity bits on line 258 (the informationbits are not used). The CLDPC codeword and parity bits are demultiplexed260 to produce the four groups of information bits and parity check bitsillustrated in FIG. 10. These four groups of bits are multiplexed intoquarter partitions 262 to produce four groups of bits on lines 264, 266,268 and 270 that are mapped to the BL, BU, ML and MU quarter-partitions.Diversity delays 272 and 274 are added to the bits for the BU and BLpartitions.

FIG. 11 starts with one systematic semi-codeword (e.g., B, R=⅔),permutes the same information bits, and re-encodes them to produce newparity check bits. Using the same information bits with these paritybits forms the complementary semi-codeword M, where B is the complementof M.

FIG. 12 is a schematic representation of another example of codewordpartitioning using Technique 1 and a rate ⅖ CLDPC code. It is similar tothe partitioning of FIG. 6, except that it generates a higher rate CLDPCcode (R=⅖ instead of R=⅓) which is more spectrally efficient. In thisexample, a codeword 280 includes 4608 information bits and 6912 paritycheck bits. The information bits are divided into four groups of 1152bits each. The parity bits are divided into four groups of 1728 bitseach. One of the groups of information bits and one of the groups ofparity check bits are mapped to each BL, BU, ML and MUquarter-partition. The bits are selected such that they formnonsystematic rate ⅘ non-catastrophic semi-codewords identified as bitsencompassed by ovals 282, 284, 286 and 288. These semi-codewords can betransmitted in the in-band on-channel radio signal illustrated in FIG.2, which includes 70-kHz upper and lower sidebands, each having 191subcarriers. The information throughput is approximately 100 kbps. Thesemi-codewords can be combined at a receiver to produce a composite rate⅖ codeword.

FIG. 13 is a block diagram of an example of CLDPC forward errorcorrection (FEC) signal flow. In this example, 4608 information bits areinput on line 290 and are encoded using a rate ⅖ code generator 292 toproduce a CLDPC codeword having 11520 bits on line 294. The CLDPCcodeword is demultiplexed 296 to produce the four groups of informationbits and parity check bits illustrated in FIG. 12. These four groups ofbits are multiplexed into quarter partitions 298 to produce four groupsof bits on lines 300, 302, 304 and 306 that are mapped to the BL, BU, MLand MU quarter-partitions. Diversity delays 308 and 310 are added to thebits for the BU and BL quarter-partitions.

FIG. 14 is a schematic representation of another example of codewordpartitioning using Technique 2 and a rate ⅖ LDPC code. It is similar tothe partitioning of FIG. 8, except that it generates a lower-rate CLDPCcode. In this example, a codeword 320 includes 4608 information bits and6912 parity check bits. The information bits are divided into two groupsof 2304 bits each, which are replicated to produce four groups of 2304bits each. The parity bits are divided into four groups of 1728 bitseach. One of the groups of information bits and one of the groups ofparity check bits are mapped to each BL, BU, ML and MUquarter-partitions. The bits are selected such that they form systematicrate 4/7 non-catastrophic semi-codewords identified as bits encompassedby ovals 322, 324, 326 and 328. These semi-codewords can be transmittedin the in-band on-channel radio signal illustrated in FIG. 3, whichincludes 98-kHz upper and lower sidebands, each having 267 subcarriers.The information throughput is approximately 100 kbps. The semi-codewordscan be combined at a receiver to produce a composite rate 2/7 codeword.As an alternative embodiment, the partitioning can start with a rate 4/7code and the information bits can be permuted to produce additionalparity bits, in the same manner as shown in FIG. 10 and FIG. 11.

FIG. 15 is a block diagram of an example of CLDPC forward errorcorrection (FEC) signal flow. In this example, 4608 information bits areinput on line 330 and are encoded using a rate ⅖ code generator 332 toproduce a CLDPC codeword having 11520 bits on line 334. The CLDPCcodeword is demultiplexed 336 to produce the four groups of informationbits and parity check bits illustrated in FIG. 14. These four groups ofbits are multiplexed into quarter partitions 338 to produce four groupsof bits on lines 340, 342, 344 and 346 that are mapped to the BL, BU, MLand MU quarter-partitions. Diversity delays 348 and 350 are added to thebits for the BU and BL quarter-partitions.

FIG. 16 is similar to FIG. 6, except that it uses a nonsystematic CLDPCcode where the information bits in the codeword are replaced by paritycheck code bits. FIG. 16 is a schematic representation of an example ofcodeword partitioning of a rate ⅓ CLDPC code using Technique 1. In thisexample, a codeword 320 includes 13824 parity check bits. The paritybits are divided into four groups of 3456 bits each. One of the groupsof parity check bits is mapped to each BL (backup lower), BU (backupupper), ML (main lower) and MU (main upper) partitions. The bits areselected such that they form nonsystematic rate ⅔ non-catastrophicsemi-codewords identified as bits encompassed by ovals 322, 324, 326 and328. These semi-codewords can be transmitted in the in-band on-channelradio signal illustrated in FIG. 4, which includes 84-kHz upper andlower sidebands, each having 229 subcarriers. The information throughputis approximately 100 kbps. The semi-codewords can be combined at areceiver to produce a composite rate ⅓ codeword. In this example,semi-codeword L includes one half of the bits of each of semi-codewordsB and M, and semi-codeword U includes the other half of the bits of eachof semi-codewords B and M.

FIG. 16 also shows 2-dimensional overlapping codeword partitioning.Frequency diversity is provided by the upper and lower sidebands, thattransmit semi-codewords U and L. Time diversity is provided byMain/Backup semi-codewords M and B. BL, BU, ML and MU representoverlapped quarter-partitions. Each quarter-partition contains half of 2semi-codewords. Each codeword bit is assigned to BL, BU, ML or MU. Thecomposite codewords are composed of semi-codewords as: B=BL+BU, M=ML+MU,L=BL+ML, and U=BU+MU. Semi-codeword B can be transmitted after M withabout a 4 sec diversity delay. The semi-codeword B is comprised of BLand BU.

Table 1 summarizes the example LDPC codes used in the variousembodiments described above.

TABLE 1 Comparison of Potential LDPC Codes Bandwidth and Full Code Semi-Systematic information code Gen code semi- Option bit rate rate raterate codewords* 1 84 kHz 1/3 1/3 2/3 No 99.2 kbps 2 84 kHz 1/3 1/2 2/3Yes 99.2 kbps 2a 84 kHz 1/3 1/2** 2/3 Yes 99.2 kbps 3 70 kHz 2/5 2/5 4/5No 99.2 kbps 4 98 kHz 2/7 2/5 4/7 Yes 99.2 kbps *Systematicsemi-codewords result in redundant information bits in the full code.The encoder replicates these redundant information bits, and the decodercombines redundant information soft-bits before decoding. Then the codegenerator rate is greater than the full code rate because of thisredundancy. **Rate-1/2 code generated using rate-2/3 generator withinformation-bit permutation and subsequent removal of 1 set ofinformation bits

FIG. 17 is a simplified functional block diagram of an FM IBOC receiver370. The receiver includes an input 372 connected to an antenna 374 anda tuner or front end 376. A received signal is provided to ananalog-to-digital converter and digital down converter 378 to produce abaseband signal at output 380 comprising a series of complex signalsamples. The signal samples are complex in that each sample comprises a“real” component and an “imaginary” component, which is sampled inquadrature to the real component. An analog demodulator 382 demodulatesthe analog modulated portion of the baseband signal to produce an analogaudio signal on line 384. The digitally modulated portion of the sampledbaseband signal is next filtered by sideband isolation filter 386, whichhas a pass-band frequency response comprising the collective set ofsubcarriers f₁-f_(n) present in the received OFDM signal. Filter 388suppresses the effects of a first-adjacent interferer. Complex signal418 is routed to the input of acquisition module 416, which acquires orrecovers OFDM symbol timing offset, or error and carrier frequencyoffset or error from the received OFDM symbols as represented in thereceived complex signal. Acquisition module 416 develops a symbol timingoffset Δt and carrier frequency offset Δf, as well as status and controlinformation. The signal is then demodulated (block 392) to demodulatethe digitally modulated portion of the baseband signal. Then the digitalsignal is deinterleaved by a deinterleaver 394, and decoded by a Viterbidecoder 396. A service demultiplexer 398 separates main and supplementalprogram signals from data signals. A processor 4002 processes the mainand supplemental program signals to produce a digital audio signal online 402. The analog and main digital audio signals are blended as shownin block 404, or the supplemental program signal is passed through, toproduce an audio output on line 406. A data processor 408 processes thedata signals and produces data output signals on lines 410, 412 and 414.The data signals can include, for example, a station information service(SIS), main program service data (MPSD), supplemental program servicedata (SPSD), and one or more advanced application services (AAS).

In practice, many of the signal processing functions described above, aswell as the functions of the transmitter and receiver can be implementedusing one or more processors or other components. Such components mayinclude integrated circuits. As used herein, the term processor includesone or more processors or other components that are programmed orotherwise configured to perform the described functions.

The embodiments described above relate the use of complementary lowdensity parity check codes in an FM IBOC radio system. However, itshould be understood that the complementary low density parity checkcodes can also be used in or with an AM IBOC radio system, a singlecarrier radio signal, or other digital signals.

While the present invention has been described in terms of severalembodiments, it will be understood by those skilled in the art thatvarious modifications can be made to the described embodiments withoutdeparting from the scope of the invention as set forth in the claims.

What is claimed is:
 1. A method comprising: constructing complementarylow density parity check codewords by generating a first codeword havinga first code rate; and partitioning the first codeword by assigninggroups of bits of the first codeword to four quarter-partitions, whereineach of the quarter partitions includes bits in one half of one of fourindependently decodable semi-codewords each having a second code ratethat is larger than the first code rate.
 2. The method of claim 1,wherein the first codeword comprises information bits and parity checkbits, and wherein the groups of bits assigned to each of the fourquarter-partitions include a group of the information bits and a groupof the parity bits.
 3. The method of claim 1, further comprising:transmitting bits of first and second ones of the fourquarter-partitions on subcarriers in a lower sideband of an in-bandon-channel signal; and transmitting bits of and third and fourth ones ofthe four quarter-partitions on subcarriers in an upper sideband of thein-band on-channel signal.
 4. The method of claim 1, wherein the firstcodeword comprises a systematic codeword.
 5. The method of claim 1,wherein the first codeword comprises a non-systematic codeword.
 6. Themethod of claim 1, further comprising: performance checking thesemi-codewords; and repartitioning the first codeword if thesemi-codewords do not meet predetermined performance criteria.
 7. Amethod comprising: constructing complementary low density parity checkcodewords by generating a first semi-codeword including information bitsand first parity bits; permuting and re-encoding the information bits ofthe first semi-codeword to produce second parity check bits and forminga second semi-codeword including the information bits and second paritybits; generating a third semi-codeword from the information bits of thefirst semi-codeword plus a first half of the parity bits from each ofthe first semi-codeword and the second semi-codeword; and generating afourth semi-codeword from the information bits of the firstsemi-codeword plus a second half of the parity bits from each of thefirst semi-codeword and the second semi-codeword.
 8. The method of claim7, further comprising: assigning groups of bits of the first, second,third and fourth semi-codewords to each of four quarter-partitions,wherein each of the quarter partitions includes bits in one half of oneof the first, second, third and fourth codewords.
 9. The method of claim7, further comprising: transmitting bits of first and second ones of thefour quarter-partitions on subcarriers in a lower sideband of an in-bandon-channel signal; and transmitting bits of third and fourth ones of thefour quarter-partitions on subcarriers in an upper sideband of thein-band on-channel signal.
 10. The method of claim 7, furthercomprising: performance checking the first and second semi-codewords;and repartitioning the parity bits of the second semi-codeword if thefirst and second semi-codewords do not meet predetermined performancecriteria.
 11. A receiver for receiving a digital radio signal, thereceiver comprising: an input for receiving a radio signal including atleast one carrier, the carrier being modulated by plurality ofinformation bits representing audio information and/or data encoded in aplurality of complementary low density parity check codewords includinga first codeword having a first code rate; wherein the first codeword ispartitioned by assigning groups of bits of the first codeword to fourquarter-partitions, and wherein each of the quarter partitions includesbits in one half of one of four independently decodable semi-codewordseach having a second code rate that is larger than the first code rate;and a processor for producing an output signal in response to thereceived radio signal.
 12. The receiver of claim 11, wherein: the firstcodeword comprises information bits and parity check bits; and thegroups of bits assigned to each of the four quarter-partitions include agroup of the information bits and a group of the parity bits.
 13. Thereceiver of claim 11, wherein: first and second ones of the fourquarter-partitions are received on subcarriers in a lower sideband of anin-band on-channel signal; and second and third ones of the fourquarter-partitions are received on subcarriers in an upper sideband ofthe in-band on-channel signal.
 14. The receiver of claim 11, wherein thefirst codeword comprises a systematic codeword.
 15. The receiver ofclaim 11, wherein the first codeword comprises a non-systematiccodeword.